Electrodes implanted in the body for electrical cardioversion or pacing of the heart are well known. More specifically, electrodes implanted in or about the heart have been used to reverse (i.e., defibrillate or cardiovert) certain life threatening arrhythmias, or to stimulate contraction (pacing) of the heart, where electrical energy is applied to the heart via the electrodes to return the heart to normal rhythm.
The sick sinus syndrome and symptomatic AV block constitute the major reasons for implantation of cardiac pacemakers today. Cardiac pacing may be performed by the transvenous method or by electrodes implanted directly onto the ventricular epicardium. Transvenous pacing may be temporary or permanent. In temporary transvenous pacing an electrode catheter is introduced into a peripheral vein and fluoroscopically positioned against the endocardium. Permanent transvenous pacing is performed under sterile surgical conditions. An electrode is positioned in the right ventricle or atrium through a subclavian vein, and the proximal terminals are attached to a pacemaker which is implanted subcutaneously.
Leads may be unipolar or bipolar. A unipolar lead system contains a single electrode in direct contact with the cardiac tissue which is connected to the negative terminal of the pacemaker with the second electrode positioned remotely, and usually consisting of the metallic pacemaker housing. In a bipolar lead system, the electrodes are in close proximity to each other and are situated within or on the heart.
In the past the surface areas of the tip electrodes have been relatively large. Typically, the electrodes have been at least equal in surface area to the cross section of the casing near the lead tip. In some applications, such as where patch type leads are used, the surface area of the electrode is much larger than the surface area associated with the diameter of the casing. In order to deliver an adequate charge to accomplish pacing of the heart, the entire surface area of the electrode must be charged to a certain level. Larger surface areas require larger amounts of energy. Energy is now an issue as the pulse generators are implanted subcutaneously within the patient. Health care costs are under constant downward pressure and insurance companies are trying to contain costs. Pacemaker replacement costs are less with a pulse generator having longer battery life since patients do not have to undergo operational procedures as often. Patients also benefit from longer battery lives, since a longer battery life means a longer time between hospital visits.
U.S. Pat. No. 5,405,373 discloses a distal end electrode having a tip with a surface which has a diameter equal to the diameter of the casing or base near the distal end electrode. In U.S. Pat. No. 5,405,373 the external surface of the electrode head is coated with a highly resistive insulating material, such as diamond-like carbon, which is deposited on the tip surface of the distal end electrode. The insulative layer is thin enough so that the threshold value is not affected. In other words, the coating or layer of insulating or dielectric material does not cause a standoff that would cause an increase in the pacing threshold. Depositing the layer of diamond-like carbon is both difficult and costly. This results in a pacing lead that is difficult and expensive to manufacture.
There is a need for a high impedance pacing lead that has a tip that is smaller in diameter than the casing near the electrode tip. Such an electrode tip would result in the use of less energy and increased battery life. There is also a need for a tip that accommodates eluting anti-inflammatory drugs. There is also a need for a pacing lead that maintains existing manufacturing techniques without adding an expensive process and new bio-materials into the human body.